Currently, there are roughly four kinds of manufacturing techniques in Micro Electric Mechanical System (MEMS) applied for inertia sensor: surface micromachining, bulk micromachining, LIGA process, and other micromachining techniques.
Wherein, the surface micromachining is to apply thin film deposition and etching technique of semiconductor process to manufacture MEMS elements on chips. As shown in FIG. 1, the steps for constructing suspension structure by surface micromachining may be classified as follows:
(a) Depositing isolation layer 2 upon silicon wafer 1.
(b) Depositing sacrificial layer 3 upon isolation layer 2.
(c) Etching sacrificial layer 3 using lithography process.
(d) Depositing a suspension structure layer 4 upon the sacrificial layer 3.
(e) Generating suspension structure layer 4 by removing the sacrificial layer 3.
The bulk micromachining is applying etching techniques, such as: anisotropic etching, etch-stop technology and etching mask, etc. to etch single crystal silicon to fabricate MEMS elements. As shown in FIG. 2, the steps for bulk micromachining are classified as follows:
(a) Depositing thin film layer 2 upon wafer 1.
(b) Etching thin film layer 2 by lithography process.
(c) Etching the silicon wafer 1 to generate suspension structure layer 21.
LIGA technique applies the combination processes of X-ray etching, micro plating, and injection molding to manufacture microstructure with high aspect ratio. The micromachining process is to apply the techniques of cutting machining, micro electrostatic discharge machining, or injection molding, etc., to manufacture MEMS elements.
When applying aforementioned traditional MEMS techniques to inertia sensor design, there are two confronting bottlenecks: first, the microstructure is with high aspect ratio; second, a lateral sensing and signal driving objectives must be achieved by means of small intervals within the microstructure. Currently, the bulk micromachining is mostly applied in the design of inertia sensor, but such kind of designing manner is always incurred with the inaccurate alignment in the crystal direction of single crystal silicon substrate and the limitation in etching width. In addition, if the lateral sensing or signal driving arrangement is required, the arrangement of lateral electrodes is another troublesome problem.
As for existent etching techniques, such as: deep reactive ion etching (RIE), bulk silicon anisotropic etching, LIGA, etc., there are several shortcomings incurred:
1. For deep RIE, it is applying two major gases to protect the side wall and to etch at the same time, such that the purpose to etch the materials vertically is reached, but there are some inherent limitations on such kind of manufacturing manner technique: first, the etched materials must be silicon based, such that the purpose for protecting the side wall may be reached; if the size difference of each zone to be etched is large, the etched depths will be very differentiated as well, so it is impossible to reach equal-depth etching; in addition, although the micro intervals can be generated by the etching processes, it is impossible for other techniques to manufacture electrodes on side surfaces.
2. For bulk silicon anisotropic etching, it mostly applies the different etching speeds of etching liquid on the crystal lattice of single crystal silicon to reach the purpose of anisotropic etching, so the etching and non-etching areas defined prior to the etching process become a crucial factor for the accurate alignment on the original crystal lattice of single crystal silicon; furthermore, controlling the etching uniformity of entire wafer is also a big problem.
3. For LIGA process, it combines the lithography, electroforming , and molding to manufacture microstructure with high accuracy and high aspect ratio; the standard LIGA technique applies synchrotron radiation X ray as lithography, and the accuracy of the microstructure may reach sub-micro level, but it is expensive and complicated, so it has developed a trend for applying ultraviolet light, laser, or plasma as light source for LIGA-like technique and, since UV lithography process incorporated with thick film photo-resist technique may realize an UV-LIGA process of low cost.
The design structure of inertia sensing system made by common MEMS technique is mainly comprised of driving, sensing, and mass block parts. For current MEMS technique, an IC thin film process is preferably adopted, but the MEMS element manufactured by IC thin film process has extremely small bearing limitation for mechanical stress, so only static products subjected none or small stress have developed, such as: acceleration gauge, force sensor, and physic sensor combined with bio-medical sensing chip, etc. In the future, MEMS will march into the field of dynamic system, so how to promote the strength for sensing signals and how to control different sensing levels have become a very difficult challenge. Therefore, developing a high aspect ratio structure, increasing mass on suspension structure layer, and arranging electrodes on side faces are crucial factors in manufacturing an inertia micro-sensor.
As shown in FIG. 3 and FIG. 4, two inertia sensing systems made by MEMS are illustrated, where in a bulk micromachining process is applied to form the suspension structure layers 21a, 21b and the inertia mass blocks 22a, 22b. The inertia mass blocks 22a, 22b are arranged below the suspension structure layers 21a, 21b. Since inertia mass blocks 22a, 22b are made of single crystal and non-conductive materials, so the suspension structure layers 21a, 21b and the inertia mass blocks 22a, 22b can only reciprocate up and down and can not be applied in lateral sensing or signal driving arrangement.